The present invention relates to a precast, modular spar system and method for constructing same for deep water oil and gas exploration, drilling, production, and storage. The spar system supports a production deck above the sea level and a riser system connecting a subsea well installation on the sea floor with the production deck. The riser system extends through a central longitudinal passageway in the spar.
An important part of the world's production or oil and gas is derived form offshore wells. While the early offshore oil and gas fields were in relatively shallow water, the need to develop oil fields in deep water has become more important as the shallow water oil and gas fields become depleted. As such, many deep water basins throughout the world have been opened to oil and gas exploration and drilling.
The original deep water applications used large drilling platforms such as concrete gravity based structures. However, as the depth increased, alternative platform methods were proposed such as steel jacket type structures fixed to and resting upon the sea floor, guyed towers, or tensioned leg platforms. Tensioned leg platforms are floating structures used in medium to deep water and in calm to rough seas. The tensioned leg platform is held below its normal buoyancy level by vertical steel mooring lines or tethers. Control of its movement in the waves and currents is similar to that of a seaway marker buoy held by a tight cable with just enough freedom to allow limited horizontal movement. The tension leg platform concept was first used in 1984 as a steel structure in about 147 meters of water and is currently being used in about 350 meters of water.
An alternative method is the floating production systems which is used in deep water or in shallow waters that are isolated from production export facilities. The floating production system drills and completes wells and contains the tools necessary to operate the subsea system. Components are assembled on the floating production system and installed remotely by a subsea vehicle. Wells pumps the heavy crude oil to jumpers which are attached to a central manifold. A floating production, storage and off loading vessel receives the crude oil from the manifold and performs initial processing and storing of the crude oil. The crude oil is off loaded to a shuttle tanker for delivery and final processing at a refinery. Another proposal is a free standing riser system which can be used in medium to deep water. Wells are drilled and completed within a subsea template. A free standing riser carries the individual flow lines that exit the riser just below sea surface. Flexible lines connect the riser to a semi-submersible production platform.
Recently, the world's first metal production spar was installed in the Gulf of Mexico to develop an oil and gas field in the deep waters of the Gulf of Mexico, some 90 miles off the coast. A spar is a deep draft floating caisson or hollow cylindrical structure similar to a buoy. Like a buoy, a spar floats and is moored or anchored to the sea floor. Spars have been used for decades as marker buoys and for gathering oceanographic data.
Although spars have been used in the past to store oil, this new production spar is the first to be used to support a production deck with buoyant well risers through the center passageway. Oil and gas gathered from wells drilled on the sea floor will be processed to pipeline quality and transported to shore. The metal spar has two main sections: the hull and the production deck. The hull is a hollow cylindrical metal structure 705' long, 72' in diameter, and weighing 12,640 tons. The hull was manufactured in Finland and shipped across the Atlantic Ocean aboard heavy lift vessels as two separate sections until reaching the Gulf of Mexico. There, the two separate sections of the spar were brought back to shore and welded together at a shipyard. The entire welded hull was then towed horizontally to the project site and upended to the vertical position by filling its lower ballast tanks with water. About ninety percent of the spar structure is below sea level with about fifty five feet above sea level to support the three story production deck and facility. The metal spar is moored in almost 2,000 feet of water by a series of chains and cables to six piles, each sunk 180 feet into the sea floor. Production risers from the subsea well are threaded through the center passageway of the spar. The production deck is a three level deck designed to accommodate 25,000 barrels of oil per day and 30 MMcf of gas per day. Facilities and crew living quarters are located on top of the floating hull section.
In general, each of the current oil and gas production systems have benefits, but also significant disadvantages. Most can only be used only within its specific application. And, although the spar is considered less expensive than the other typical production systems to develop a field in almost 2,000 feet of water, it still requires a coordinated international team effort to construct, ship, assemble, and tow to the production site.